Water-absorbing resins are widely used in sanitary goods, hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed “superabsorbent polymers,” or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,559,335, the disclosures of which are incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.
As used here and hereafter, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, i.e., particles containing from no water up to an amount of water less than the weight of the particles. The terms “SAP gel” or “SAP hydrogel” refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water.
The dramatic swelling and absorbent properties of SAPs are attributed to (a) electrostatic repulsion between the charges along the polymer chains, and (b) osmotic pressure of the counter ions. It is known, however, that these absorption properties are drastically reduced in solutions containing electrolytes, such as saline, urine, and blood. The polymers function much less effectively in the presence of such physiologic fluids.
The decreased absorbency of electrolyte-containing liquids is illustrated by the absorption properties of a typical, commercially available SAP, i.e., sodium polyacrylate, in deionized water and in 0.9% by weight sodium chloride (NaCl) solution. The sodium polyacrylate can absorb 146.2 grams (g) of deionized water per gram of SAP (g/g) at 0 psi, 103.8 g of deionized water per gram of polymer at 0.28 psi, and 34.3 g of deionized water per gram of polymer of 0.7 psi. In contrast, the same sodium polyacrylate is capable of absorbing only 43.5 g, 29.7 g, and 24.8 g of 0.9% aqueous NaCl at 0 psi, 0.28 psi, and 0.7 psi, respectively. The absorption capacity of SAPs for body fluids, such as urine or menses, therefore, is dramatically lower than for deionized water because such fluids contain electrolytes. This dramatic decrease in absorption is termed “salt poisoning.”
The salt poisoning effect has been explained as follows. Water-absorption and water-retention characteristics of SAPs are attributed to the presence of ionizable functional groups in the polymer structure. The ionizable groups typically are carboxyl groups, a high proportion of which are in the salt form when the polymer is dry, and which undergo dissociation and solvation upon contact with water. In the dissociated state, the polymer chain contains a plurality of functional groups having the same electric charge and, thus, repel one another. This electronic repulsion leads to expansion of the polymer structure, which, in turn, permits further absorption of water molecules. Polymer expansion, however, is limited by the crosslinks in the polymer structure, which are present in a sufficient number to prevent solubilization of the polymer.
It is theorized that the presence of a significant concentration of electrolytes interferes with dissociation of the ionizable functional groups, and leads to the “salt poisoning” effect. Dissolved ions, such as sodium and chloride ions, therefore, have two effects on SAP gels. The ions screen the polymer charges and the ions eliminate the osmotic imbalance due to the presence of counter ions inside and outside of the gel. The dissolved ions, therefore, effectively convert an ionic gel into a nonionic gel, and swelling properties are lost.
The most commonly used SAP for absorbing electrolyte-containing liquids, such as urine, is neutralized polyacrylic acid, i.e., containing at least 50%, and up to 100%, neutralized carboxyl groups. Neutralized polyacrylic acid, however, is susceptible to salt poisoning. Therefore, to provide an SAP that is less susceptible to salt poisoning, either an SAP different from neutralized polyacrylic acid must be developed, or the neutralized polyacrylic acid must be modified or treated to at least partially overcome the salt poisoning effect.
The removal of ions from electrolyte-containing solutions is often accomplished using ion exchange resins. In this process, deionization is performed by contacting an electrolyte-containing solution with two different types of ion exchange resins, i.e., an anion exchange resin and a cation exchange resin. The most common deionization procedure uses an acid resin (i.e., cation exchange) and a base resin (i.e., anion exchange). The two-step reaction for deionization is illustrated with respect to the desalinization of water as follows:NaCl+R—SO3H−R—SO3Na+HClHCl+R—N(CH3)3OH−R—N(CH3)3Cl+H2O.The acid resin (R—SO3H) removes the sodium ion; and the base resin (R—N(CH3)3OH) removes the chloride ions. This ion exchange reaction, therefore, produces water as sodium chloride is adsorbed onto the resins. The resins used in ion exchange do not absorb significant amounts of water.
The most efficient ion exchange occurs when strong acid and strong base resins are employed. However, weak acid and weak base resins also can be used to deionize saline solutions. The efficiency of various combinations of acid and base exchange resins are as follows:
Strong acid—strong base (most efficient)
Weak acid—strong base
Strong acid—weak base
Weak acid—weak base (least efficient).
The weak acid/weak base resin combination requires that a “mixed bed” configuration be used to obtain deionization. The strong acid/strong base resin combination does not necessarily require a mixed bed configuration to deionize water. Deionization also can be achieved by sequentially passing the electrolyte-containing solution through a strong acid resin and strong base resin.
A “mixed bed” configuration of the prior art is a physical mixture of an acid ion exchange resin and a base ion exchange resin in an ion exchange column, as disclosed in Battaerd U.S. Pat. No. 3,716,481. Other patents directed to ion exchange resins having one ion exchange resin embedded in a second ion exchange resin are Hatch U.S. Pat. No. 3,957,698, Wade et al. U.S. Pat. No. 4,139,499, Eppinger et al. U.S. Pat. No. 4,229,545, and Pilkington U.S. Pat. No. 4,378,439. Composite ion exchange resins also are disclosed in Hatch U.S. Pat. Nos. 3,041,092 and 3,332,890, and Weiss U.S. Pat. No. 3,645,922.
The above patents are directed to nonswelling resins that can be used to remove ions from aqueous fluids, and thereby provide purified water. Ion exchange resins used for water purification must not absorb significant amounts of water because resin swelling resulting from absorption can lead to bursting of the ion exchange containment column.
Ion exchange resins or fibers also have been disclosed for use in absorbent personal care devices (e.g., diapers) to control the pH of fluids that reach the skin, as set forth in Berg et al. U.S. Pat. No. 4,685,909. The ion exchange resin is used in this application to reduce diaper rash, but the ion exchange resin is not significantly water absorbent and, therefore, does not improve the absorption and retention properties of the diaper.
Ion exchange resins having a composite particle containing acid and base ion exchange particles embedded together in a matrix resin, or having acid and base ion exchange particles adjacent to one another in a particle that is free of a matrix resin are disclosed in B. A. Bolto et al., J. Polymer Sci.: Symposium No. 55, John Wiley and Sons, Inc. (1976), pages 87-94. The Bolto et al. publication is directed to improving the reaction rates of ion exchange resins for water purification and does not utilize resins that absorb substantial amounts of water.
Other investigators have attempted to counteract the salt poisoning effect and thereby improve the performance of SAPs with respect to absorbing electrolyte-containing liquids, such as menses and urine. For example, Tanaka et al. U.S. Pat. No. 5,274,018 discloses an SAP composition comprising a swellable hydrophilic polymer, such as polyacrylic acid, and an amount of an ionizable surfactant sufficient to form at least a monolayer of surfactant on the polymer. In another embodiment, a cationic gel, such as a gel containing quaternized ammonium groups and in the hydroxide (i.e., OH) form, is admixed with an anionic gel (i.e., a polyacrylic acid) to remove electrolytes from the solution by ion exchange. Quaternized ammonium groups in the hydroxide form are very difficult and time-consuming to manufacture, thereby limiting the practical use of such cationic gels.
Wong U.S. Pat. No. 4,818,598 discloses the addition of a fibrous anion exchange material, such as DEAE (diethylaminoethyl) cellulose, to a hydrogel, such as a polyacrylate, to improve absorption properties. The ion exchange resin “pretreats” the saline solution (e.g., urine) as the solution flows through an absorbent structure (e.g., a diaper). This pretreatment removes a portion of the salt from the saline. The conventional SAP present in the absorbent structure then absorbs the treated saline more efficiently than untreated saline. The ion exchange resin, per se, does not absorb the saline solution, but merely helps overcome the “salt poisoning” effect.
WO 96/17681 discloses admixing discrete anionic SAP particles, such as polyacrylic acid, with discrete polysaccharide-based cationic SAP particles to overcome the salt poisoning effect. Similarly, WO 96/15163 discloses combining a cationic SAP having at least 20% of the functional groups in a basic (i.e., OH) form with a cationic exchange resin, i.e., a nonswelling ion exchange resin, having at least 50% of the functional groups in the acid form. WO 96/15180 discloses an absorbent material comprising an anionic SAP, e.g., a polyacrylic acid and an anion exchange resin, i.e., a nonswelling ion exchange resin.
Water-absorbing resins, particularly superabsorbent polymers, have been in use in disposable, absorbent fibrous articles, such as diapers and bandages for many years. These superabsorbent polymers have been used together with a batt of absorbent fibers, such as cellulose fibers, used to absorb and hold the liquid within the product, and for faster liquid uptake during the slower absorption of the liquid by an adjacent superabsorbent polymer. The most common absorbent batt used in the diaper art is manufactured from fluffed wood pulp fibers, as disclosed in U.S. Pat. No. 2,788,003, hereby incorporated by reference. A densified paper-like surface layer also has been used in conjunction with an absorbent batt to improve “wicking” of the liquid to the absorbent batt, as disclosed in U.S. Pat. Nos. 3,612,055 and 3,938,522. The absorbent structure of the present invention is useful with or without absorbent fibers and/or wicking layers disclosed in the above-identified patents.
Others have attempted to manufacture a continuous roll of woven or non-woven fibrous material that contains a high percentage, e.g., 50-80% by weight, of a particulate superabsorbent polymer, such as sodium polyacrylate. Examples of fibrous substrates impregnated with superabsorbent polymer are found in U.S. Pat. Nos. 5,614,269; 5,980,996; and 5,756,159 wherein a fibrous substrate is impregnated with the superabsorbent polymer by impregnation with the monomer and subsequent polymerization by contact with UV light for polymerization in situ while in contact with the fibrous substrate.
Other patents, including U.S. Pat. Nos. 5,607,550 and 5,997,690 teach the continuous manufacture of a fibrous substrate containing more than about 50% superabsorbent particles (50-60%) by the wet, papermaking process. In accordance with the wet process. The raw material, including fibers and superabsorbent particles, are mixed with copious quantities of water, or other liquid medium capable of swelling the superabsorbent particles, and deposited onto a water-pervious support member, generally a Fourdinier wire, where much of the water is removed leaving a wet mass of fiber and superabsorbent polymer particles. The wet mat is transferred from the pervious support member and consolidated under heat and pressure to form the fibrous substrate having the superabsorbent particles distributed throughout. As disclosed in U.S. Pat. No. 5,997,690, sufficient absorbency performance requires at least about 50% by weight superabsorbent particles based on the total weight of the absorbent article. The most difficult problem encountered in attempting to continuously manufacture the sheet material containing a relatively high percentage of superabsorbent particles is in achieving structural integrity of the article both during and after manufacture without significant loss (shakeout) of superabsorbent particles.
Another principal process for continuously making a consolidated sheet of material is a “dry” process. In a dry process, filler material, such as cellulosic fibers, is coated with a resin binder in a gaseous stream, or by mechanical means. For example, the fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) may be coated with a thermosetting synthetic resin, such as a phenol-formaldehyde resin, in a blowline blending procedure, wherein the resin is blended with the fiber with the aid of air turbulence. Thereafter, the resin-coated fibers from the blowline are subjected to pre-press drying, for example, in a tube-like dryer, and then are randomly formed into a mat by air conveying the fibers onto a support member (e.g., a forming wire). The formed mat, preferably having a moisture content of less than about 10 wt. %, is then pressed under heat and pressure in a press between a pair of heated platens to cure the thermosetting resin and to compress the mat into an integral consolidated structure. The consolidated structure may be embossed on an outer surface by texturing one of the press platens to achieve a desired embossed design in the outer surface of the product during consolidation.
Another process for continuously manufacturing a consolidated sheet of material is a wet-dry process, wherein resin-blended fiber from the blowline is mixed with water as the conveying medium and is formed into a mat as a wet slurry on a water-pervious support member where water is removed by mechanical means to a moisture content of about 60% or less. The formed mat then is mechanically conveyed through a multi-deck air dryer in which the moisture content is further reduced to about 10% or less. The mat is then pressed under heat and pressure similar to the above-described “dry” process.
In accordance with the principles of the present invention, it has been found, unexpectedly, that a continuous sheet having a combination of acidic and basic water-absorbing resin particles that are essentially unneutralized (0% to about 25% neutralized) can be continuously manufactured on conventional papermaking apparatus, using any of the above-described wet, dry, or wet-dry processes to manufacture a water-absorbent sheet-like substrate containing 50%-100% by weight of the combination of acidic and basic water-absorbent particles, added to form the sheet material articles of the present invention as separate acidic and basic resin particles, or as multicomponent particles containing both the acidic and basic resins. The sheet materials can be manufactured having new and unexpected structural integrity with little or no shakeout or loss of superabsorbent particles during or after manufacture.
The sheet materials of the present invention exhibit exceptional water absorption and retention properties, especially with respect to electrolyte-containing liquids, and thereby overcome the salt poisoning effect. In addition, the sheet materials have an ability to absorb liquids quickly, demonstrate good fluid permeability and conductivity into and through the SAP particles, and have a high gel strength such that the hydrogel formed from the SAP particles, upon hydration, resists deformation under an applied stress or pressure, when used alone or in a mixture with other water-absorbing resins.